Filaments in incandescent lamps are commonly manufactured in the form of coils (or coiled coils) by winding the filament wire in a coil around a metallic mandrel, annealing the coil, and selectively dissolving the mandrel from the coil with a suitable acid. In order to produce high quality filaments, care must be taken to avoid embrittlement of the filament wire, commonly tungsten, by diffusion of extraneous metal, such as iron, from the mandrel into the filament, typically during the procedure in which the filament is annealed.
Probably the most commonly employed material for mandrels is steel; however, substantial amounts of iron often diffuse into the filament wire during the manufacture and seriously impair its useful quality. Efforts have been made to avoid this diffusion by coating the steel with a non-alloying metal which impairs the diffusion of iron into the filament wire. U.S. Pat. No. 3,662,789 discloses the use of copper as such a coating. Similarly, tungsten may be employed as such a coating.